Vascular inflammation in obesity and sleep apnea

Abstract

Background: Unrecognized obstructive sleep apnea (OSA) is highly prevalent in obesity. Both obesity and OSA are associated with vascular endothelial inflammation and increased risk for cardiovascular diseases. We investigated directly whether the endothelial alterations that are attributed commonly to obesity are in fact related to OSA.

Methods and results: Seventy-one subjects with a body mass index ranging from normal to obese underwent attended polysomnography. To assess vascular inflammation and oxidative stress directly, we quantified the expression of nuclear factor-kappaB and nitrotyrosine by immunofluorescence in freshly harvested venous endothelial cells. To evaluate basal endothelial nitric oxide (NO) production and activity, we quantified the expression of endothelial NO synthase (eNOS) and phosphorylated eNOS. Vascular reactivity was measured by brachial artery flow-mediated dilation. Expression of eNOS and phosphorylated eNOS and flow-mediated dilation were significantly lower, whereas expression of nitrotyrosine was significantly greater in OSA patients (n=38) than in OSA-free subjects (n=33) regardless of central adiposity. Expression of nuclear factor-kappaB was greater in obese OSA patients than in obese OSA-free subjects (P=0.004). Protein expression and flow-mediated dilation were not significantly affected by increasing body mass index or central obesity in OSA patients and in OSA-free subjects. After 4 weeks of continuous positive airway pressure therapy, flow-mediated dilation and expression of eNOS and phosphorylated eNOS significantly increased whereas expression of nitrotyrosine and nuclear factor-kappaB significantly decreased in OSA patients who adhered to continuous positive airway pressure >/=4 hours daily.

Conclusions: Untreated OSA rather than obesity is a major determinant of vascular endothelial dysfunction, inflammation, and elevated oxidative stress in obese patients.

Figure 1

Expression of endothelial nitric oxide synthase (eNOS), phosphorylated eNOS (P-eNOS), nitrotyrosine and nuclear factor kappa B (NFκB) in venous endothelial cells obtained from patients with obstructive sleep apnea (OSA) and OSA-free subjects. Patients with OSA (n=38, red boxplots) and OSA-free subjects (n=33, white boxplots) were stratified as being normal weight (N), overweight (OV), or obese (OB) by body mass index (BMI), and as having central obesity by increased (I) versus normal (N) waist circumference or waist to hip ratio. Expression of eNOS and P-eNOS was lower whereas expression of nitrotyrosine was greater in OSA patients compared with OSA-free subjects for all central adiposity categories. Expression of eNOS and P-eNOS was lower in obese OSA patients than in obese OSA-free subjects. Expression of NFκB was significantly greater in OSA patients who were obese or had central obesity than in their OSA-free counterparts. Expression of eNOS, P-eNOS, nitrotyrosine, and NFκB was similar within all adiposity categories in OSA patients and in OSA-free subjects (p=NS for all). Data are shown as boxplots with median values and Q1-to-Q3 represented by horizontal black lines and the upper and lower bounds of the box, respectively. Whisker tips extend to the most extreme value within 1.5 times the Q1-to-Q3. Outliers are not shown. au indicates arbitrary units.

Figure 2

Relationship between brachial artery flow-mediated dilation (FMD) and adiposity. Brachial artery FMD was lower in patients with obstructive sleep apnea (OSA) (red boxplots) who were normal-weight compared with OSA-free subjects (white boxplots) with similar body habitus measurements. Non-significant trend toward lower FMD was observed in overweight OSA patients compared with overweight OSA-free subjects. Brachial artery FMD was lower in patients with OSA who had normal (N) or increased (I) waist circumference and normal waist to hip ratio compared with OSA-free subjects with similar adiposity. Brachial artery FMD was similar in OSA-free subjects who were normal-weight (N), overweight (OV) and obese (OB), and/or had normal (N) or increased (I) waist circumference and waist to hip ratio (p=NS for all). Brachial artery FMD was also similar in OSA patients who were normal-weight, overweight and obese, and/or had normal or increased waist circumference and waist to hip ratio (p=NS for all). Data are shown as boxplots with median values and Q1-to-Q3 represented by horizontal black lines and the upper and lower bounds of the box, respectively. Whisker tips extend to the most extreme value within 1.5 times the Q1-to-Q3.

Figure 3

Relationship between endothelial expression of nitric oxide synthase (eNOS), phosphorylated eNOS (P-eNOS), nitrotyrosine and nuclear factor kappa B (NFκB and the severity of obstructive sleep apnea assessed by the apnea-hypopnea index (AHI) after adjustment for age, gender, and body mass index. Expression of eNOS and P-eNOS correlated inversely with AHI whereas expression of nitrotyrosine and NFκB correlated directly with AHI. P values reflect the linear associations between protein expression and AHI using generalized additive models with loess smoothers for continuous variables.

Figure 4

Effects of continuous positive airway pressure (CPAP) therapy on protein expression in venous endothelial cells. Expression of endothelial nitric oxide synthase (eNOS) and phosphorylated eNOS (P-eNOS) increased whereas expression of nitrotyrosine and nuclear factor kappa B (NFκB) decreased in patients with OSA who adhered with CPAP≥4 hours daily after adjustment for age, gender, and body mass index (n=19).